With the advance of mobile communication, location-based services become more popular. Existing location-based services consider the absolute location (for example, the latitude and longitude) of service providers, identifying (for instance, on a cellular phone) items based on distance measures, and a person can receive certain advertisements or information based on his or her resident address. Such existing approaches require the accurate location of the mobile devices (as well computers), and rely on a mature communication infrastructure and sophisticated subscription system for support. However, such approaches can lack the ability to enable location-based services in an ad hoc manner. Additionally, existing message delivery mechanisms are not suitable for many applications due to high cost and limitations on scalability in terms of end users.